The development of means of storing bulk quantities of electrical power has become increasingly important in recent years. Redox cells, having soluble electrodes in both of the charged and discharged states, have been the object of increased interest as a method for the efficient storage of electrical energy. Such redox cells could store energy generated from time dependent energy sources such as solar electric and windmill electric installations. Examples of redox cells which can be used in connection with the present invention are described in detail in the technical paper "Electrically Recharbable Redox Flow Cells" by Lawrence H. Thaller, NASA TM X-71540. Additionally, such flow cells are described in U.S. Pat. No. 3,996,064 which is hereby incorporated by reference. In their simpliest embodiments, these redox flow cells contain two storage tanks, each containing one of the two metal ions, together with chloride anions which make up the redox couple. Although almost any redox couple can be used in such a cell, considerations regarding efficiency can be made in the choice of the particular couple. Thus, systems such as the Fe.sup.+2 /Fe.sup.+3 //Ti.sup.+3 /TiO.sup.+2 and the Fe.sup.+2 /Fe.sup.+3 //Cr.sup.+3 /Cr.sup.+2 systems are preferred. Thus, the cathodic fluid,. e.g., aqueous concentrated Fe.sup.+3, and the anodic fluid, e.g., aqueous concentrated Ti.sup.+3, are fed from their respective tanks through the redox flow cell where the system can either be charged from an external source or can be discharged to release the stored electrical energy. The power that can be withdrawn from or put back into the system depends on many factors including the tank volumes, the flow rates and the electrochemical features of the particular redox couple utilized and the characteristics of the electrode compartments.
In a two tank system employing multiple passes of the fluid, the fluid would constantly be recycled after passage through the fuel cell. In a four tank system, the fluids would pass from their respective tanks through the fuel cell and then into two other storage tanks. The system could then be electrically recharged by applying a suitable voltage to the terminals of the power conversion section as the fluids are pumped back up to the original tanks.
A two-tank system is shown in the drawing. The anodic fluid and cathodic fluid tanks are represented by 1 and 2, respectively. Pipelines 3 for the feeding operation to the redox flow cell are provided from the tanks. The redox flow cell 4 contains inert electrodes 5 and a selective membrane 6. After passing through the cell, the fluids are returned to tanks 1 and 2 by pumps 7.
The costs of the electrical accumulator installations have been the subject of much interest as indicated by the technical paper "Cost and Size Estimates for an Electrochemical Bulk Energy Storage Concept" by Marvin Warshay and Lyle O. Wright, NASA TM X-71805. Deployment of the electrical accumulator system utilizing a redox cell system with a solar power source has been the subject of the technical paper "The Redox Flow System for Solar Photovoltaic Energy Storage" by Patricia O'Donnel, Randall F. Gahn and William Pfeiffer, NASA TM X-73562.
U.S. Ser. No. 707,124, filed July 20, 1976, of which the inventor of the present application is co-inventor, describes the use of hydrochloric acid-silica gels as an ion transport medium for use in the membrane separating the couple compartments of such redox flow cells.
The membrane utilized must provide an impermeable barrier to the cations of the particular couple utilized. However, the membranes must be permeable to the extent of maintaining the charge neutrality of each compartment by the migration of the anion used through the membrane. It should be noted that fuel cells can be designed with either anions or cations migrating through the membrane to provide the neutralization required. However, there is an inherent disadvantage from an energy standpoint in moving cations from the anode compartment during discharge as opposed to moving anions from the cathode compartment. That is, if a hydrogen cation is required to migrate during discharge, one mole of hydrochloric acid is required per Faraday over and above any acid that may be required for pH adjustment needed for solution stabilazation.
Therefor, redox fuel cells that use anion migration through the membrane are somewhat preferred. Anions that can be used with the gels of the present invention include halide ions such as chloride and bromide.
Materials that have been used as anion-permeable membranes include polymers such as IONAC 3475. Such membranes can be made by grinding a quaternary ammonium ion-exchange resin to a powder and then polymerizing a monomer in the present of the thus-formed powder. Additionally, a web can be used in conjunction with the membranes to provide support.
Among the prior art membranes are those described in U.S. Pat. No. 3,497,389 issued to Carl Berger et al. These membranes utilize inorganic additives of controlled water vapor characteristics capable of retaining water and providing water vapor pressures above 100.degree. C. The additives can be mixed with the ion conducting material, e.g., zirconium phosphate, granulated and pressed into discs which are then sintered.
Further, prior art structures include ceramic membranes such as those described in U.S. Pat. No. 3,392,103 issued to Carl Berger.
Other separators for use in different types of batteries have been described in U.S. Pat. No. 2,816,154 to Mendelsohn and U.S. Pat. No. 3,018,316 to Higgins.
An object of the present invention is a membrane that can be used in bulk electrical energy storage systems which has a low ionic resistivity, high selectivity for anions as opposed to metal cations and low electronic conductivity.
A further object of the invention is a membrane for use in a redox flow cell which exhibits minimal increases in resistivity with the passage of time and which prevents the accumulation of unequal amounts of water between the two sides of the redox cells.